Electrical apparatus



April 29, 1952 ALFORD 2,594,839

ELECTRICAL APPARATUS Filed March 29, 1946 INVENTOR ANDREW ALFORDATTORNEY Patented Apr. 29, 1952 ELECTRICAL APPARATUS Andrew Alford,Cambridge, Mass, assignor to the United States of America as representedby the Secretary of War Application March 29, 1946, Serial No. 657,953

6 Claims.

This invention relates generally to electrical apparatus and moreparticularly to antennas adapted for use with radio frequency directionfinding systems.

Some of the more recent types of radio frequency direction findingsystems not only sweep in azimuth but also employ a frequency scanningreceiver. Such a direction finding system provides a presentationshowing both the azimuth and the carrier frequency of received signals,thus increasing the intelligence obtainable from radio frequencytransmitters. In these direction finding systems, the antennas employedmust meet rigid requirements. It is desirable that these antennas notonly be operable over a wide range of frequencies but that they alsomaintain a substantially constant response pattern, with regard toshape, as the operating frequency is varied. Generally, the responsepattern of an antenna varies in shape or directivity as the operatingfrequency is varied.

It is an object of this invention to provide an antenna particularlyadapted for use with radio frequency direction finding systems.

It is also an object to provide an antenna having a response pattern,which remains constant with regard to shape, over a wide range ofoperating frequencies.

It is also an object to provide an antenna which has a unidirectionalresponse pattern.

It is a further object to provide an antenna which is physically smallwith respect to a wavelength corresponding to the operating frequencyand which has a low value of input impedance.

An antenna which accomplishes the above objects consists of a sleeveddipole with a sheet reflector. The dipole elements are center fed andtheir central portions are enclosed by a sleeve. The ends of the dipoleelements which extend beyond the ends of the sleeve, are enlarged toapproximately the size of the sleeve. The function of this dipole is toexcite the sleeve rather than to serve as a radiator which is the normalfunction of a conventional dipole.

Rather than being aligned end to end along a common axis like theconventional dipole, the two elements of the dipole in this inventionare bent to form a V. A sheet reflector is placed behind the sleeveddipole so that the direction of response is in the direction of the apexof the V formed by the radiators.

Other objects, features and advantages of this invention will suggestthemselves to those skilled in the art and will become apparent from thefollowing description of the invention taken in connection with theaccompanying drawing in which Fig. l is an isometric view of an antennaembodying the principles of this invention and;

Fig. 2 is a top view of the apparatus shown in Fig. 1.

Referring now tothe drawing, a bent dipole consisting of elements l0 andH, is center fed by a balanced transmission lineconsisting of coaxialcables !2 and Hi. The inner conductor of cable I2 is connected to dipoleelement in, and similarly, the inner conductor of cable I3 is connectedto dipole element II. The outer conduc tors of cables i2 and I3terminate in contact with a bent sleeve It which encloses the centralportion of the dipole. The extremities of dipole elements It and H,which protrude beyond the ends of sleeve M, are enlarged so that theyare approximately the same size as sleeve 14.

A sheet reflector 15, bent in the general shape of the letter W, isoriented behind the sleeved dipole with the inner legs of reflector I5generally parallel With respective dipole elements IB and II.

For purposes of explanation, the antenna will be considered astransmitting rather than receiving. Generally, antennas have the samereception characteristics as transmitting characteristics.

Dipole elements I!) and H are center fed and are excited by equal andopposite voltages. The enlarged end of element [0 radiates a smallamount of energy but most of its energy is used to excite sleeve M. Themajor function of sleeve M is that of a radiator. Similarly, dipoleelement H excites the other half of sleeve M which also radiates energy.Being connected at its midpoint, to the outer conductors of cables l2and 13, the sleeve l4 remains at ground potential at its midpoint.However, the two ends of sleeve l-l become electrically charged due tothe presence of dipole elements It! and II and sleeve l4 contributesmost of the radiated energy fromthis antenna system.

Since the circumference of sleeve $4 is large with respect to itslength, there is close coupling between space and the sleeve. Beingclosely coupled to space, radio frequency waves traveling longitudinallyalong sleeve [4, due to excitation by the dipole, suffer appreciableattenuation in transit. This attenuation phenomenon isnot noticeable inconventional thin wire antennas where the diameter of the radiator issmall com pared to its length. Consequently, with the conventional thinwire dipole, maximiun radiation occurs in a direction perpendicular tothe longitudinal axis of the dipole. With the antenna herein disclosed,the direction of maximuinradiation does not occur in a directionperpendicular to the radiator. Accordingly, the radiator of the antennais bent until the lobe patterns of each half of sleeve i l occur in thedesired direction. With sleeve 14 bent to form an included angle of 120degrees, the two forward lobes of each half of sleeve M add togethercausing maximum radiation in the direction of the apex of the antenna, asheet reflector has been added to the array.

The length of this antenna is not critical with respect to frequency,however, to obtain good performance, the length of the dipole should beat least one-fourth of a wavelength long corresponding to the lowestoperating frequency.

The antenna shown in the drawing is fed by a balanced transmission line,however, it could readily be adapted for use with an unbalanced line,such as a single coaxial transmission line, by the use of a balun. Abalun is an impedance balancing device which permits apparatus which isat an impedance balance with respect to ground, to be connected toapparatus which is unbalanced with respect to ground.

a While there has been here described what is at present considered tobe the preferred embodiment of the invention, it will be obvious tothose skilled in the art that various changes and modifications may bemade therein without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:

1. A broad band horizontally polarized antenna including a center feddipole having a length of the order of one-fourth of a wavelengthcorresponding to the lowest operating frequency with the outerextremities of said dipole enlarged and with said dipole bent to form aV whose ineluded angle is of the order of 120 degrees, said dipolecomprising a pair of elements each having inner and outer ends, ametallic sleeve with a rectangular cross-section bent to form a Vconcentrically enclosing the central portion of said dipole, a feed linecomprising a first coaxial transmission cable and a second coaxial transmission cable with the inner conductor of said first cable insertedthrough a hole in said sleeve and connected to one end of one element ofsaid dipole and with the inner conductor of said second cable insertedthrough a hole in said sleeve and connected to one end of the otherelement of said dipole and with the outer conductors of said cablesterminated in contact with said sleeve and a sheet reflector bent toform a W oriented behind said dipole so that the inner legs of saidreflector are parallel with respective elements of said dipole wherebythe direction of maximum response of said antenna is in the direction ofthe apex of the V formed by said sleeve and whereby the response patternof said antenna is substantially constant in shape over a wide range ofoperating frequencies.

2. A broad band plane polarized antenna including a sleeve radiator bentto form a V, a bent, center fed, two-element, exciting dipole insertedcoaxially in said sleeve with the ends of said dipole protruding out ofsaid sleeve and with said ends enlarged to approximately the diameter ofsaid sleeve, a W-shaped sheet reflector oriented behind said sleeveradiator with the inner legs of said W reflector substantially parallelto respective elements of said dipole so that the direction of maximumresponse of said antenna is in the direction of the apex of the V formedby said radiator, a two conductor transmission line insorted through ahole in saidradiator andconnected to the two halves of said dipolewhereby the response pattern of said antenna is substantially constantin shape over a wide range of operating frequencies.

. 3. A broad band plane polarized antenna in- I eluding a sleeveradiator bent to form a V,

a W-shaped sheet reflector, means for supporting said sleeve radiator infront of said reflector with the arms of said V substantially parzillelto the inner arms of said W, the outer arms of said W- shaped reflectoreach extending forwardly and transversely to the direction of one ofsaid arms of said sleeve radiator, and means for exciting said sleeveradiator at the two ends thereof in phase opposition.

4. Apparatus as claimed in claim 3 wherein the diameter of said sleeveradiator is a large fraction of its length thereby causing said radiatorto be closely coupled to space.

5. A broad band antenna comprising a hollow sleeve radiator bent to forma V, a W-shaped sheet reflector, means for supporting said sleeveradiator in front of said reflector with the arms of said Vsubstantially parallel to the inner arms of said W, means for excitingsaid sleeve radiator at the ends thereof comprising a first and a secondexciting element disposed coaxially with the ends of said sleeve andspaced therefrom a distance equal to a small fraction of their length,said exciting elements being of substantially the same diameter as saidsleeve and of a length that is a small fraction of the length of saidsleeve, and an electrical transmission system for supplying energy tosaid exciting elements.

6. Apparatus as described in claim 5 wherein said electricaltransmission system comprises a first and a second conductor disposedinternally and coaxially of said sleeve and electrically joined to saidfirst and second exciting element-s respectively, and a first and asecond coaxial transmission line perpendicularly joining said sleeveradiator at points near the center thereof, the outer conductors of saidtransmission line making electrical junction with said sleeve radiatorand the inner conductors of said transmission lines passing throughfirst and second openings in said sleeve radiator and making electricaljunction respectively with said first and second conductors disposedinternally of said sleeve radiator.

ANDREN ALFORD.

CETED The following references are of record in the file of this patent:

UNITED STriTE-S PATENTS Number Name Date 2,134,126 Hooven Oct. 25, 19382,167,709 Cork Aug. 1, 1939 2,181,870 Carter Dec. 5, 1939 2,204,175Carter June 11, 1940 2,224,898 Carter Dec. 17, 1940 2,287,220 AlfordJune 23, 1942 2,311,364 7 Buschbeck et al. Feb. 16, 1943 2,404,196"Seeley July 16,1946 2,419,552 Himmel et al Apr. 29, 1947 OTHERREFERENCES Proceedings of I. R. E. November 1940, page 518.

